In a drum mixer of the type to which this invention relates, the mixing drum is rotatably mounted on an elongated rigid frame which often comprises a trailer chassis that provides for transporting the mixer from one job site to another. The drum itself is substantially cylindrical and axially elongated, its length being typically on the order of 40 ft. (12 meters). The rear end of the drum, which is its inlet or charging end, is usually at a higher elevation than its front or discharging end so that the drum axis is inclined at an angle of about 21/2.degree. to 31/2.degree. to the horizontal.
In the type of drum mixer here under consideration, the drum is surrounded by two coaxial trunnion engaging rings or so-called tires, one near its charging end and the other near its discharging end. Each of these tires has a plainly cylindrical peripheral surface concentric to the drum axis, and has flat front and rear surfaces lying in planes normal to the drum axis. The drum is both supported and driven for rotation by means of two pairs of trunnion rollers, one pair for each tire, the two trunnion rollers of each pair being spaced to opposite sides of the drum axis. Each trunnion roller has a plainly cylindrical peripheral surface that rollingly engages the cylindrical periphery of its tire.
For reasons explained below, the trunnion rollers must be individually adjustable in horizontal directions transverse to their axes. To that end each trunnion roller is rotatably mounted on its own base structure, and each base structure is slidably adjustable on the frame. Each base structure carries a gear box through which its trunnion roller is driven from an electric motor or the like. Ordinarily there are two electric motors for drum rotation, one for each pair of trunnion rollers, each motor being mounted on the frame between its trunnion rollers. On the exterior of each gear box there is a driving sheave that has a belt connection with the adjacent motor.
It will be apparent that the drum must be maintained at an axial location at which each of its tires is fully engaged with the pair of trunnion rollers for that tire. That location is defined by means of front and rear thrust rollers which are freely rotatable on vertical axes and which are mounted on the mixer frame directly below the drum axis for cooperation with the front tire. The front thrust roller rollingly engages the flat front surface of the front tire; the rear one similarly engages the flat rear surface of that tire.
If the axes of the four trunnion rollers extended fore-and-aft exactly parallel to the vertical plane containing the drum axis, the drum would have a tendency to be moved axially forward by gravity as it rotated, owing to the rearward and upward inclination of its axis. Although the front thrust roller would prevent the drum from moving so far forward that the tires would be disengaged from the trunnion rollers, the high thrust force that the drum and its contents would impose upon that thrust roller would soon cause excessive wear of both it and the front tire.
To avoid this, the four trunnion rollers are adjusted so that the axis of each, although extending horizontally, is skewed slightly out of parallelism with the vertical plane that contains the drum axis. By reason of this skewed orientation of the trunnion roller axes, each trunnion roller, as it rotates, imparts a small component of axial driving force to the drum in the "uphill" direction, in addition to the substantially large component of rotational driving force that it imparts to the drum. Since all of the trunnion rollers rotate in the same direction, all of their axes should have the same direction and amount of skew, which is to say that all four trunnion roller axes should be parallel to one another and slightly oblique to the vertical plane containing the drum axis. If the axes of the trunnion rollers are excessively skewed, the rear thrust roller will be overloaded. Thus the amount of such skew should be just enough for the forward force of gravity upon the drum to be counterbalanced by the rearward force due to the trunnion rollers, so that the drum imposes no substantial load upon either of the thrust rollers.
Heretofore there has been no satisfactory method or means of ensuring accurate adjustment of the orientations of the trunnion roller axes, and in particular there has been no reliably accurate means for checking parallelism of the trunnion roller axes. The lack of such an expedient has had expensive consequences. If the axes of the four trunnion rollers are not parallel, then the trunnion rollers impose unequal axial forces upon the drum and in effect work against one another. This condition results in scuffing of the peripheral surfaces of the trunnion rollers and the tires, and it can occasion the need for replacement of those parts after a relatively short period of operation. Replacement of a trunnion roller is no small matter in terms of cost of the roller, labor involved in making the replacement, and down time on the mixer; but replacement of a drum tire is much more expensive because it requires that the drum be cut apart.
Heretofore the usual expedient for trunnion adjustment has been to use a tape measure to measure from an edge of the mixer frame to marks designating the centers of the trunnion roller shafts. Because of difficulties and inaccuracies attending the use of a tape measure, correct adjustment of all four trunnion rollers was difficult to achieve and was likely to be as much a matter of luck as of skill. The result was that the trunnion rollers and tires on a drum mixer had a useful life that was often as short as two years and was rarely equal to the useful life of the rest of the machine.
Adjustment of the trunnion rollers is not a one-time requirement but must take place rather frequently during the life of a drum mixer. The angle of tilt of the drum axis to the horizontal is adjusted by raising or lowering one end of the mixer frame and should be established in accordance with the type of material being mixed and the rate at which it must pass through the drum. Any change in this tilt will of course require a change in the angle of skew of the trunnion roller axes.
Proper adjustment of the trunnion rollers is complicated by the fact that the effect of their axial skew varies with the weight of the load in the mixer. Somewhat paradoxically, if the orientation of the trunnion roller axes is such that the front tire bears lightly against the front thrust roller when the drum is empty, that tire will bear against the rear thrust roller when the drum is operating under full load. Apparently the weight of the load urges the drum into more firm engagement with the trunnion rollers, so that there is less slippage between them and the respective tires, with the result that the "uphill" component of driving force that the rollers exert upon the drum increases to a greater extent than the "downhill" force that gravity exerts upon the loaded drum and its contents.
Since a normal load weighs substantially more than the drum itself, the usual procedure is to so adjust the trunnion rollers, with the drum running under full normal load, as to bring the front tire into light engagement with the rear thrust roller. With such an adjustment properly made, the front tire will lightly engage the front thrust roller when the drum is empty.
It is not easy for an unskilled person to understand and visualize the forces that the drum and the trunnion rollers exert upon one another and to appreciate the kind and amount of adjustment that must be given to each individual trunnion roller during an adjustment procedure. Furthermore, the persons performing an adjustment procedure have heretofore been guided through it only by their own knowledge and skill rather than by mechanical constraints imposed by the involved apparatus. Because of these factors, mistakes were frequent, even when adjustments were made by skilled and conscientious personnel.